Apparatus for minimizing stress in a heated semiconductor filament

ABSTRACT

STRESSES ARE RELIEVED IN A HEATED SEMICONDUCTOR FILAMENT BY A SYSTEM THAT INCLUDES MEANS FOR POSITIONING THE FILAMENT HOLDERS WITH RESPECT TO EACH OTHER. A SEMICONDUCTOR FILAMENT IS SUPPORTED AT OPPOSITE ENDS BY HOLDERS WHICH ARE ADJUSTABLY MOUNTED TO THE END PLATES OF A DEPOSITION CHAMBER. THESE ADJUSTABLE MOUNTINGS MAY TAKE THE FORM OF A BELLOWS ATTACHED TO THE END PLATE AND FASTENED TO AN ELECTRODE WHICH CONNECTS A SOURCE OF ELECTRICAL ENERGY TO THE FILAMENT HOLDER. AS THE FILAMENT EXPANDS DUE TO HEATING, ONE OF THE ELECTRODES MOVES, AND THIS MOVEMENT   IS SENSED BY A MOTION TRANSDUCER WHICH GENERATES A SIGNAL PROPORTIONAL TO THE ELECTRODE MOVEMENT. THE OUTPUT OF THE MOTION TRANSDUCEER DRIVES A POWER OPERATOR TO POSITION ONE OF THE TWO ELECTRODES TO ALLOW FOR THE FILAMENT EXPANSION. THUS, THE STRESSES IN THE FILAMENT DUE TO EXPANSION ARE MINIMIZED.

Jan. 26, 1 971 L. D. DYER 3,558,281

APPARATUS FOR MINIMIZING STRESS IN A HEATED SEMICONDUCTOR FILAMENT Filed July 11, 1968 2 Sheets-Sheet 1 l 4 I CONVERTER 1 I I 24 /:-lo

I I 4 I, I l 7'2 2/ 1 Fl 6. l

INVENTOR ATTORNEY LAWRENCE 0. DYER I D. DYER 3,558,231 APPARATUS FOR MINIMIZING STRESS IN A HEATED Jan. 26 1971 SEMICONDUCTOR FILAMENT Filed July 11 1968 2 Sheets-Sheet 2 m 2 \Z: M J a W m w k\\ k\\ 1% .1 I l 6 M 3 w F.

3,558,281 APPARATUS FOR MINIMIZING STRESS IN A HEATED SEMICONDUCTOR FILAMENT Lawrence D. Dyer, Richardson, Tex., assignor to Texas Instruments Incorporated, Dallas, Tex., a corporation of Delaware Filed July 11, 1968, Ser. No. 744,028 Int. Cl. B01j 17/00 US. Cl. 23-273 16 Claims ABSTRACT OF THE DISCLOSURE Stresses are relieved in a heated semiconductor filament by a system that includes means for positioning the filament holders with respect to each other. A semiconductor filament is supported at opposite ends by holders which are adjustably mounted to the end plates of a deposition chamber. These adjustable mountings may take the form of a bellows attached to the end plate and fastened to an electrode which connects a source of electrical energy to the filament holder. As the filament expands due to heating, one of the electrodes moves, and this movement is sensed by a motion transducer which generates a signal proportional to the electrode movement. The output of the motion transducer drives a power operator to position one of the two electrodes to allow for the filament expansion. Thus, the stresses in the filament due to expansion are minimized.

This invention relates to a system for relieving stress, and more particularly to a system for relieving stress in a heated semiconductor filament.

The vapor reduction of silicon and germanium halides onto a hot filament so as to obtain the pure semiconductor in an elemental form is a process well known to those active in semiconductor technology. Typical of such processes is that described in the copending application of Lawrence D. Dyer et al., Ser. No. 689,289, filed Dec. 11, 1967, and assigned to the assignee of the present invention. The technique described in the above application provides for growing a crystal on a silicon substrate by first hot vapor etching the substrate and then depositing silicon. The hot vapor etching is carried out at about 1325 C. in an atmosphere of a gaseous mixture containing hydrogen and hydrogen chloride. Following the etching step, silicon is deposited on the filament by exposure to an environment containing hydrogen and trichlorosilane. Following reduction of the hydrogen chloride concentration, the temperature of the filament is lowered to about 1200 C. Silicon is deposited upon the filament in this manner for a period sufiicient to produce a rod of desired diameter.

One problem with crystal growth in the above manner arises from the longitudinal expansion of the filament while it is being heated from the ambient temperature to the etch and deposition temperature. Such expansion can lead to bowing of the filament and to irregularly deposited rods, which gives rise to expensive handling problems during technological processing. For monocrystalline filaments where it is desired to deposit a single crystal, the expansion may cause a stress increase that generates undesirable dislocations in the finished crystal.

Basically, a deposition chamber in which the above process may be completed includes a cylindrical shaped enclosure with two end plates each of which provides a means for supporting the filament in a desired position within the chamber. Electrical current for heating the filament to the etch and deposition temperatures is supplied by means of electrodes passing through the end plates and connected to the filament and an electrical power source. Heretofore, one technique for attaching the filament to the electrodes was by means of a two part United States Patent slip-chuck which was designed to minimize stressing of the heated filament by relative movement of the two parts. Unfortunately, due to the heavy current flow required for filament heating (on the order of 400 amperes), welding between the parts of the slip-chuck often occurred, causing the chuck to freeze in one position. The filament was then supported by two chucks each having a fixed position and expansion of the rod caused undesirable stress.

In accordance with this invention, there is provided a system for minimizing the stress in a heated semiconductor filament which includes filament holders for supporting both ends of the filament in a deposition chamber. The filament holders are adjustably mounted to maintain the heated filament in a given position within the chamber. A motion transducer senses the movement of one of the filament holders and generates a signal proportional to the amount of movement. This signal activates a power operator which positions one of the two filament holders to return the transducer signal to an initial value, thereby relieving the stress in the heated filament.

In accordance with a specific embodiment of this invention, there is provided a system for maintaining a controlled stress during crystal growth on a heated filament. There are some semiconductor applications where the complete absence of dislocations hinders the diffusing of dopants. In such cases, the system of this invention may be adjusted to maintain a present level of stress in the crystal.

It is an object of this invention to provide a system for relieving the stress in a heated filament. Another object of this invention is to provide a system for supporting a heated filament in a minimum stress condition. A further object of this invention is to provide a system for supporting a heated semiconductor filament in a deposition chamber. Yet another object of this invention is to provide a system for supporting a heated semiconductor filament in a minimum stress condition during vapor reduction. Still another object of this invention is to provide a system for adjustably supporting a heated semiconductor filament in accordance with the movement of a filament holder. An additional object of this invention is to provide a system for supporting a heated semiconductor filament in a minimum stress condition'by measuring the stress and adjusting the filament supports.

A better understanding of the invention and its advantages will be apparent from the specification and claims and from the accompanying drawings illustrative of the invention.

Referring to the drawings:

FIG. 1 illustrates a reaction chamber and a feedback loop for maintaining a heated semiconductor filament in a minimum stress condition;

FIG. 2 is a view of the upper half of the deposition chamber of FIG. 1 showing an alternate system for positioning the upper filament holder; and

FIG. 3 is a view of the lower half of the deposition chamber showing an alternate embodiment of a motion transducer and filament holder positioner.

Referring initially to FIG. 1, there is shown a deposition chamber including a quartz tube 10 and two end plates 12 and 14. The top end plate 12 includes an inlet .pipe 16 for admitting reaction gases into the chamber during the etching and crystal growth steps. An outlet pipe 18 is provided in the lower end plate 14 to permit exit of the gases. The inlet pipe 16 or the outlet pipe 18 also provides a means for evacuating the deposition chamber to remove absorbed gases at the completion of the etch and deposition phases. Since contamination may come from materials used within the chamber, it is important to select only materials which will withstand the temperatures and reaction gases without giving up contaminants.

The semiconductor filament 20, onto which the silicon or germanium halides are deposited, is supported within the deposition chamber by means of an upper chuck 22 and a lower chuck 24. The lower chuck 24 may be of the slip-chuck design to provide one means of minimizing stress in the filament 20 as it is heated to the etching and deposition temperatures. Typically, the chucks 22 and 24 are made from a hard graphite material provided with a tapering bore for gripping the ends of the filament.

The lower chuck 24 is slip-fit mounted to an electrode 26 which provides a means for supplying electrical current to the filament 20 for heating thereof. Electrical energy from a source (not shown) is supplied to the chuck 24 through a path that includes a flexible conductor 28 attached to the electrode by means of a ring clamp 30. A compression nut 32 threaded onto a tube 34 provides a means for holding the electrode 26 in an initial position as determined by the length of the filament 20. To maintain the electrode below the temperature at which it gives oif contaminants, a stream of water is directed into a center bore of the electrode by means of a pipe 36. The assembly of the chuck 24, the electrode 26, and the tube 34 are supported by the lower plate 14 through a flexible bellows 38. The lower end of the bellows 38 is attached to the end plate 14 in a manner to form a gas-tight seal therewith. Similarly, the upper end of the bellows 38 is secured to the tube 34 in a gas-tight manner. A frictionless bearing 40 threads into the lower plate 14 to position the tube 34 to maintain the lower chuck 24 in alignment with the upper chuck 22. As illustrated, this frictionless hearing may be an air operated device, and as such couples to an air supply by means of a pipe 42. Air bearings of the type shown are commercially available and their operation well understood. In the manner employed here, the bearing 40 aligns the chuck 24 and in addition provides for longitudinal movement of the tube 34 with a minimum of friction.

At the upper end of the deposition chamber, the upper chuck 22 is threaded onto an electrode 44 which provides a means for supplying electrical current to the filament 20 for heating thereof during the etch and deposition phases of the vapor reduction process. The electrode 44 connects to a source of electrical energy (not shown) through a flexible conductor 46 attached to the electrode by means of a ring clamp 48. The electrode 44 is held in position by means of a tube 50. A flexible bellows 52 provides the connecting link between the assembly of the upper chuck 22, the electrode 44, and the tube 50 and the upper plate 12. Note, the bellows 52 is considerably shorter than the bellows 38. The attachment of the bellows 52 to the end plate 12 provides a gas-tight seal; the bellows also attaches to the tube 50 in a gas-tight manner. A frictionless bearing 54, similar to the bearing 40, threads into the end plate 12 and provides a means for aligning the upper chuck 22 with the lower chuck 24. Fluid pressure for operation of the bearing 54 is supplied by means of a pipe 56 and exits to the atmosphere through a pipe 58.

To position the filament for the vapor reduction process, it is first inserted into the chuck 22 which has been threaded onto the electrode 44. The lower chuck 24 slips over and grips the lower end of the filament 20*. With the electrode 26 positioning the chuck as explained previously, a stress free mounting of the filament is possible. To heat the filament 20 to the etching and deposition temperatures, electrical energy connects thereto through the electrodes 26 and 44. As the electrode temperature increases from ambient to the process temperature, the filament 20 expands lengthwise. For a 20" rod, this can amount to inch.

Expansion of the heated semiconductor filament 20 causes the electrode 26 to be displaced downward. For movement of the electrode 26 to occur, some force must be exerted by the heated filament 20 on the chuck 24. This force causes stresses to be developed within the filament which may generate undesirable dislocation content in the finished crystal. This downward displacement is sensed by a motion transducer which includes a linear differential transformer 60 having a primary winding 62 and series connected secondary windings 64 and 66. An iron core 68 is slidably positioned between the primary and secondary wnidings of the transformer 60 to change the inductive coupling between the windings. The core 68 moves in response to movement of the electrode 26 by means of a connecting link 70. Thus, as the electrode 26 moves as a result of expansion of the heated filament 20, the inductive coupling between the primary and secondary windings of the transformer 60 changes, thereby producing a change in voltage output from the transformer which is proportional to the movement of the electrode.

Coupled to the output of the transformer 60 is.an amplifier 72 which generates a signal change from an initial value that will be proportional to the amplifier input. A servo motor 74 responds to the departure of the amplifier output from the initial value. Operation of the servo motor 74 in conjunction with the amplifier-72 will be in accordance with standard techniques for such systems. As such, the speed of rotation of a shaft 76 will be proportional to the magnitude of the change in the amplifier output from the initial value. The shaft 76 drives a motion converter 78 which provides linear motion to a plunger 80. The plunger 80 positions the elec-,

trode 44 by engaging the ring clamp 48.

In operation, prior to positioning the filament 20 withinthe deposition chamber, a dial 82 on the amplifier 72 is adjusted to provide an initial offset to the electrode 44 from the at rest position of the bellows 52. The filament 20 will then be positioned in the chamber as explained previously and electrical current passed therethrough by means of the flexible conductors 28 and 46.

As the temperature of the filament 20 increases to the operating level, the filament expands and the electrode 26 is displaced axially. This axial displacement is detected by the linear difi'erential transformer 60 which generates a signal to the amplifier 72. The output of the amplifier 72 energizes the servo motor 74 thereby producing a rotation of the shaft 76 and an upward movement of the plunger 80. An upward movement of the plunger 80 causes the electrode 44 and in turn the upper chuck 22 to be displaced axially upward to compensate for the expansion of the filament 20. An upward movement of the chuck 22 relieves the stresses in the filament that originally caused the displacement of the electrode 26. The electrode 26 returns to its at rest position and the output of the differential transformer 60 returns to an initial value. The output of the amplifier 72 goes to zero and the shaft 76 comes to rest.

Referring to FIG. 2, there is shown an alternate embodiment of a system for positioning the upper chuck 22 in response to movement of the electrode 26. The same reference numbers are used in FIG. 2. for parts that appeared in FIG. 1. Thus, the end plate 12 encloses one end of the quartz tube 10. The electrode 44 is positioned within the tube 10 by means of a tube 50 and a bellows 52. An air bearing 54 aligns the electrode 44 with a fluid pressure supplied by a pipe 56. An electrical current source connects to the electrode 44 through a flexible conductor 46 and a ring clamp 48.

Fluid pressure for the air bearing 54' also flows into the bellows 52 through a passage between the bearing and the bellows. The bellows pressure will be controlled by a flow regulator 88 connected in the exit pipe 58. This flow regulator receives a control signal from the amplifier 72; it in eflect replaces the servo motor 74in the system of FIG. 1.

Instead of mechanically positioning the electrode 44 amplifier 72 thereby adjusting the flow rate through the regulator 88. The lower mechanism of the deposition chamber remains as illustrated in FIG. 1 and operates as described previously.

Movement of the electrode 26 again changes the output voltage from the differential transformer 60. This change in voltage is amplified by the amplifier 72 to adjust the flow regulator 88. A downward displacement of the electrode 26 results in a lowering of the pressure level in the bellows 52 thereby raising the electrode 44 due to the spring action of the bellows. Raising the electrode 44 relieves the stress in the heated filament 20 which originally caused movement of the electrode 26. Relieving the filament stress allows the electrode 26 to return to its original position and the output of the transformer 60 returns to an initial value.

Referring to FIG. 3, there is shown another embodiment of a stress relieving system which may be employed with the deposition chamber illustrated in FIG. 1. The electrode 26 will be positioned to support the filament 20 by means of the chuck 26 in the manner as previously explained. However, instead of a linear differential transformer to sense the motion of the electrode 26, an orifice 90 is employed instead. There will be attached to the end of the electrode 26 a positioning ring 92 which engages two set screws 94 and 96 of a ring coupler 98. The ring coupler 98' attaches to a beam 100 which travels vertically in tracks 102 and 104-. A power operator, including a cylinder 106 with a piston 108, provides the necessary power to position the beam 100. Fluid pressure for operating the cylinder 106 is supplied through a pressure control 110 by means of pipes 112 and 114. A fluid pressure signal developed by a pneumatic amplifier 118 and proportional to the back pressure on the orifice 90, controls the fluid pressure to the cylinder 106 and consequently the position of the beam 100. The back pressure on the orifice 90 is controlled by the spaced relationship between the orifice and a sensing bar 116 attached to the ring 92.

In operation, after the filament 20 has been installed in the deposition chamber by the techniques previously described, the set screws 94 and 96 are adjusted to provide a slight offset to the electrode 26. The sensing rod 116 is then adjusted to provide a nozzle back pressure which results in a pressure in the cylinder 106 to hold the beam 100 in an initial position. Note that in the embodiment shown in FIG. 3, the electrode 44- is not initially offset.

Movement of the electrode 26 changes the space relationship between the push rod 116 and the ring coupler 98. This displacement is sensed by a change in back pressure at the orifice 90 and the pressure controller 110 receives a new input signal. A new input signal to the pressure controller 110 changes the fluid pressure applied to the cylinder 106 which repositions the beam 100. Repositioning the beam 100 causes the ring coupler 98 to be moved in a direction to restore the original space relationship between the ring coupler and the sensing bar 116. When this relationship has been reestablished, the orifice back pressure will return to an initial value and the pressure controller 110 will maintain a new pressure in the cylinder 1%. Repositioning the ring coupler 98' also establishes a new spacing between the chuck 22 and the chuck 24 to minimize the stress in the heated filament 20.

Although certain specific systems for the motion transducer and power positioners have been illustrated and described, other systems are considered within the scope of the invention. For example, the linear differential transformer 60 may be replaced by a strain gauge bridge which generates an unbalanced voltage as a result of movement of the electrode 26. Also, there are other means besides flexible bellows for supporting the electrodes 26 and 44 to the end plates. For example, diaphragms (both corrugated and flat) and roll seals may be used.

While several embodiments of the invention, together with modifications thereof, have been described in detail herein and shown in the accompanying drawings, it will be evident that various further modifications are possible in the arrangement and construction of its components without departing from the scope of the invention.

What is claimed is:

1. Apparatus for controlling the stress in a heated semiconductor filament comprising:

a first chuck for supporting the semiconductor filament,

a second chuck for supporting the opposite end of the filament, means responsive to a stress developed in the semiconductor filament for generating a signal varying proportional thereto from an initial value, and

means for positioning one of said chucks relative to the other to return said signal to the initial value and thereby maintain the stress in the heated filament at a desired level.

2. Apparatus for controlling the stress in a heated semiconductor filament as set forth in claim 1 wherein said positioning means includes a power operator for positioning one of said chucks relative to the other to maintain the stress in the heated semiconductor filament at a desired level.

3. Apparatus for controlling the stress in a heated semiconductor filament as set forth in claim 2 wherein said responsive means includes a motion transducer for sensing the movement of one of said chucks relative to the other and producing a signal proportional to the stress in the semiconductor filament.

4. Apparatus for controlling the stress in a heated semiconductor filament as set forth in claim 3 wherein said sensing means includes a linear differential transformer having an output signal that varies proportional to the movement of one of said chucks relative to the other.

5. Apparatus for controlling the stress in a heated semiconductor filament as set forth in claim 3 wherein said motion transducer includes an orifice for producing a fluid back-pressure proportional to the movement of one of said chucks relative to the other.

6. Apparatus for controlling the stress in a heated semiconductor filament as set forth in claim 5 wherein said positioning means includes a fluid pressure power operator coupled to the same chuck as said motion transducer to reposition said chuck and return the fluid backpressure to an initial value.

7. Apparatus for minimizing a stress in a heated semiconductor filament comprising:

a first chuck for supporting one end of the semiconductor filament,

a second chuck for supporting the opposite end of the semiconductor filament,

means for adjustably mounting said first chuck with respect to said second chuck,

means for mounting said second chuck in a manner to permit movement thereof as a result of a stress developed in the semiconductor filament,

means responsive to the movement of said second chuck and generating a signal varying proportional thereto from an initial value, and

means for positioning said first chuck with respect to said second chuck in response to the generated signal to minimize the stress in the heated semiconductor filament and return the generated signal to the initial value.

8. Apparatus for minimizing the stress in a heated semiconductor filament as set forth in claim 7 wherein the adjustable mounting means for said first chuck and the mounting means for said second chuck includes a bellows.

.9. Apparatus for minimizing the stress in a heated semiconductor filament as set forth in claim 8 including a first electrode for supplying current to said first chuck and a second electrode for supplying current to said second chuck.

10. Apparatus for minimizing the stress in a heated semiconductor filament as set forth in claim 9 including a frictionless bearing for each of said electrodes to maintain said first and second chucks in alignment.

11. Apparatus for minimizing the stress in a heated semiconductor filament as set forth in claim 7 wherein said responsive means includes a linear differential transformer generating an electrical signal proportional to the movement of said second chuck.

12. Apparatus for minimizing the stress in a heated semiconductor filament as set forth in claim 11 wherein said positioning means includes a servo motor responsive to the output of said linear differential transformer and driving a motion converter coupled to the adjustable mounting means for said first chuck, said servo motor positioning the first chuck through the motion converter in response to the output of said linear differential transformer.

13. Apparatus for minimizing the stress in a heated semiconductor fiilarnent during a crystal growing operation in a desposition chamber comprising:

a first chuck for supporting one end of the semiconductor filament,

a second chuck for supporting the opposite end of the semiconductor filament,

a first plate including an adjustable mounting for said first chuck attached to one end of the deposition chamber to orient said first chuck in a given location in said chamber,

a second plate including a mounting for said second chuck attached to the opposite end of the deposition chamber for positioning said second chuck in a spaced relationship with said first chuck,

a first electrode passing through said first plate for connecting the first chuck to a source of electrical current,

a second electrode passing through said second plate for connecting the second chuck to a source of electrical current,

frictionless bearings attached to each of said plates around the respective electrodes to maintain said first and second chucks in proper alignment and allow for vertical movement thereof, and

means responsive to the movement of said second chuck for positioning said first chuck to relieve a stress in the heated filament.

14. Apparatus for minimizing the stress in a heated semiconductor filament during a crystal growing operation as set forth in claim 13 wherein said first and second plates include a bellows attached to each and to the respective electrodes for supporting said first and second chucks.

15. Apparatus for minimizing the stress in a heated semiconductor filament during a crystal growing operation as set forth in claim 14 wherein said responsive means includes:

a linear differential transformer responsive to the movement of said second chuck for generating an electrical signal proportional to the stress in the heated semiconductor filament, and

a servo motor responsive to the output of said linear eter adjustment for positioning said first chuck to relieve the stress in the heated semiconductor filament and return the output of the linear differential transformer to an initial value. 16. Apparatus for minimizing the stress in a heated semiconductor filament during a crystal growing operation as set forth in claim 14 wherein said responsive means includes:

an orifice for producing a fluid back-pressure proportional to the movement of said second chuck as a result of a stress in the heated semiconductor filament, and I a pressure control valve responsive to the fluid backpressure to control the pressurization of the bellows of said first plate to position the first chuck to relieve the stress in the heated semiconductor filament and return the fluid back-pressure to an initial value.

References Cited UNITED STATES PATENTS 6/1942 Vickers 60+52 3/1953 Cronstedt 33174 9/1966 Paulik et al. 7316 us. 01; X.R. 8233.31;73 l5.6

differential transformer and coupled to a microm- 

